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We report an athermal optical delay-interferometer capable of demodulating any OC-768 differential phase-shift keying (DPSK) signal on the ITU 50-GHz grid over the C+L-band. The demodulator is based on a free-space Michelson interferometer. Experiments are performed with a 42.7-Gb/s nonreturn-to-zero DPSK signal and a 42.7-Gb/s return-to-zero DPSK signal with 67% duty cycle. Receiver sensitivity (at a bit error rate of 10-9) of better than ∼35.5 dBm is achieved for both signals. Negligible temperature-induced penalty is observed over an operational temperature range between 0°C and 70°C. The penalty associated with laser frequency offset (from the ITU grid) is also investigated.